This invention relates to devices for extracting power from a fluid flow, such as a tidal stream, and structures for pumping fluids in response to such a flow.
With increasing public awareness of environmental pollution and in particular, global warming there is a growing interest in renewable energy sources. A 1994 survey of the energy available in sea or river currents and tidal streams around the UK by the Department of Trade and Industry""s renewable energy unit at Harwell [see publication reference 1], found that a considerable fraction of the UK""s energy needs could be met if this energy could be harnessed.
The energy in the currents is kinetic rather than potential, which means that it has to be extracted in a different way from that employed in a conventional hydroelectric scheme. Typically, in a tidal stream installation, a turbine might be placed underwater in the tidal stream to extract the energyxe2x80x94an underwater equivalent of a wind power generator. For example, in a development funded by the EC [2], it is planned to set up submarine propeller driven turbines in selected locations where the current flows rapidly
A disadvantage of these conventional underwater systems is that in order to access the energy of the fluid flow the moving parts are placed underwater in a hostile environment making them prone to damage and inconvenient and costly to access and repair. Furthermore, if the water is slowed too much (i.e. too big a fraction of the kinetic energy is extracted), then the head needed to drive it will be increased. To minimise the required head, thereby obviating the need for a barrage, any turbine placed in the stream will have to have its blades highly feathered, making it uneconomic.
According to one aspect of the present invention there is provided an apparatus for extracting power from a fluid flow, the apparatus comprising at least one fluid directing formation formed to define a channel having a flow accelerating constriction shaped such that fluid in the channel is caused to accelerate as it flows through the flow accelerating constriction of the channel; a fluid drivable engine disposed at a position exterior to the channel; a conduit disposed to provide fluid communication between the fluid drivable engine and a portion of the channel having an accelerated fluid flow, the fluid drivable engine being arranged such that fluid flow along the conduit acts to drive the fluid drivable engine.
The apparatus of the present invention alleviates the disadvantages of the prior art by providing a way of using the underwater fluid flow to pump fluid away from the flow so that it can be led to a fluid drivable engine, such as a turbine, sited at position remote from the underwater fluid flow. This can avoid moving parts underwater and the correspondingly high maintenance costs. Furthermore, a controllable fraction of the power in any fluid flow can be extracted. It should be noted that the apparatus will function on any scale and as such is adaptable to many different situations. This property enables the system to be produced as modules that can be combined or used alone depending on circumstances. A further advantage of this apparatus is its low environmental impact: as much of the infrastructure is underwater the only visible signs are the fluid drivable engine housing and pylons bringing the power cables.
Although the conduit can be disposed in any portion of the channel having accelerated fluid flow, preferably it is sited in a portion of the channel formed to provide a maximum fluid velocity. This arrangement provides for increased efficiency of the apparatus.
In preferred embodiments, the at least one channel is substantially symmetrical about a plane mid-way between its ends. Although an asymmetrical channel is possible, and may even be preferable for extracting power particularly, from a one way flow, a symmetrical channel allows a single construction to be used for extracting power in both directions from a two way flow, such as a tidal flow.
Advantageously, the interior surface defining the channel is generally curved. A curved profile decreases the losses due to turbulence thereby allowing a greater flow velocity for the same head of fluid.
In an alternative embodiment, a fluid reservoir and a fluid communication path between the fluid drivable engine and the fluid reservoir are provided. Although in some embodiments, fluid may be expelled from the conduit and exit via the fluid drivable engine, and in others the suction from the conduit may be used to suck air through the fluid drivable engine, in this alternative embodiment a fluid reservoir is provided so that fluid is sucked from the fluid reservoir passes through the fluid drivable engine and is expelled via the conduit into the channel
In preferred embodiments, the at least one fluid directing formation is arranged to define a plurality of channels arranged in parallel and having a corresponding plurality of conduits. The channels can be arranged in parallel in a single fluid directing formation or, alternatively, a plurality of fluid directing formations defining a plurality of channels can be arranged in parallel across the fluid flow. A plurality of channels arranged in parallel within the fluid flow allow an increase in the power extracted from a fluid flow. In addition multiplexing is simple; pipe connections to all fast streams in the vicinity can be connected in parallel to drive a single fluid drivable engine, thereby achieving economy of scale. Alternatively, a plurality of fluid drivable engines can each be arranged in fluid communication with a corresponding conduit and channel.
A multiplexed arrangement such as that described above allows small quantities of power to be extracted from a widely distributed area. This obviates the need for a large head of water, conventionally produced by a dam. Furthermore, the extraction of small quantities of power over a large area reduces the impact on existing eco-systems..
In some embodiments, a centrifugal pump having a fluid inlet and low velocity and high velocity fluid outlets is arranged in the channel such that the fluid-inlet receives fluid flowing through the channel, the low velocity fluid outlet being arranged to return fluid to the channel and the high velocity fluid outlet being arranged to expel fluid into the conduit. Thus, in this embodiment fluid exits via the conduit rather than being sucked into it.
Preferably, a generally flat circular drum with a fluid channel comprising a helix is arranged to receive fluid flowing through the channel such that fluid entering the drum forms a swirling disk and a portion of the fluid is expelled into the conduit. This arrangement allows fluid to be expelled into the conduit, without the provision of moving parts that are liable to wear and need servicing, within the fluid flow.
Advantageously, the circular drum has a double wedge shaped cross section, such that the cross section is wider at the outer circumference than it is in the middle section. This arrangement increases viscous drag in the central region and decreases the formation of vortices. In an alternative embodiment vortex formation is reduced by concentric vanes on the inside of the drum.
In preferred embodiments, the apparatus for extracting power comprises: at least two containers arranged in parallel in a fluid flow path between the channel and the fluid drivable engine, each container comprising a replenishment valve allowing fluid communication between the interior and exterior of the container; and an isolation valve arrangement such that the fluid communication between individual containers and the channel and fluid drivable engine can be inhibited, so that when a fluid contained in one container that is in fluid communication with the channel and fluid drivable engine is exhausted the isolation valve arrangement is operable to temporarily inhibit the fluid communication between the channel and fluid drivable engine via that container so that the container can be replenished using the replenishment valve. This embodiment allows an alternative fluid to that present in the fluid flow to flow between the tanks and the fluid drivable engine and to drive the fluid drivable engine. Thus, a fluid with a lower viscous drag than the fluid of the flow can be used to flow between the fluid drivable engine and tanks, reducing losses in the system. This is particularly important if the fluid drivable engine is located at some distance from the fluid flow as may be the case in, for example, tidal flows if the fluid drivable engine is located on shore. Furthermore, if the system is driving a gas turbine this arrangement acts to produce a reduced exhaust pressure for the gas turbine which increases its efficiency. It should be noted that this system is highly compatible with a gas turbine generator in which hydrocarbons are used to supplement, say tidal energy.
According to another aspect of the device there is provided a structure operable to pump fluid in response to underwater fluid flow, comprising at least one fluid directing formation formed to define a channel having a flow accelerating constriction shaped such that fluid in the channel is caused to accelerate as it flows through the low accelerating constriction of the channel; a conduit disposed to provide fluid communication between a portion of the channel having an accelerated fluid flow and a point exterior to the channel.
The structure of the present invention alleviates the disadvantages of the prior art by providing a way of using the underwater fluid flow to pump fluid away from the flow so that it can be led to a site remote from the flow, possibly an on shore site.